/Engine/Source/Runtime/Engine/Classes/GameFramework/Character.h
Z velocity is zero during walking movement, and will be here as well. Another velocity can be computed here if desired and will be used when starting to fall.
virtual void OnWalkingOffLedge_Implementation ( const FVector & PreviousFloorImpactNormal, const FVector & PreviousFloorContactNormal, const FVector & PreviousLocation, float TimeDelta )